How infectious is the hepatitis B virus? Readings from the occult

Occult HBV infection (OBI) is defined as the presence of the virus in the liver, with detectable or undetectable HBV DNA in the serum of individuals testing hepatitis B surface antigen (HBsAg) undetectable in blood, using the most sensitive commercial assays.1 OBI can be either seropositive (anti-HBc and/or anti-HBs positive) or seronegative (anti-HBc and anti-HBs negative), and when detectable, the amount of HBV DNA in the serum is usually very low (<200 IU/mL). OBI is a relatively new aspect of the natural history of HBV infection that has now been observed worldwide. Its prevalence varies according to the levels of HBV endemicity and viral genotype.2 The identification of OBI in asymptomatic blood donors and attempts by blood transfusion services worldwide to eliminate the ‘window period’ of HBV transmissibility, has prompted the introduction in many countries of nucleic acid testing (NAT) in order to detect HBV infection in blood donors.3 The study by Candotti and colleagues in GUT 4 is the latest in a series of important studies from this group addressing the challenge of the infectivity of blood products from OBI donors. Their investigation of three repeat HBsAg-negative donors from Slovenia who remained undetected for HBV DNA by highly sensitive NAT and who transmitted HBV to nine recipients by blood components, has allowed a revised estimation of the minimal HBV infectious dose from the …